Cancers of the breast, uterine corpus, and ovary accounted for 38% of the estimated 480,000 new cases of cancer (excluding basal and squamous cell cancers of the skin) that were diagnosed in U.S. women in 1987. The two constituents of oral contraceptive formulations, estrogen and progestogen, are known to play a role in both animal and human carcinogenesis. This proposal describes a series of in-depth analyses of oral contraceptive use in relation to cancer of the breast, endometrium, and ovary among women 20-54 years of age who participated in a population- based case-control study. The data, gathered by personal interviews with 4,742 women with breast cancer, 548 women with ovarian cancer, 672 women with endometrial cancer, and 4,754 controls, provide extensive details on reproductive, medical, and contraceptive histories, family history of female cancers, demographic factors, and health care utilization. Information on breast cancer stage, and histologic information on ovarian and endometrial cancers, are also available. The major aims are: (1) to delineate the specific groups of women for whom oral contraceptive (OC) use substantially lowers the risk of endometrial and ovarian cancer, and to determine whether, among the subgroup(s) of women so identified, the effect is related to OC formulation or to patterns of use by age, reproductive history, or other factors; (2) to estimate the net effect of OC use of cancer of the breast and reproductive tract through 54 years of age; (3) to estimate the percentage and number of breast cancer cases in the U.S. that can be attributed to menstrual and reproductive factors through 54 years of age; (4) to continue investigation of the possibility that OC use might influence the risk of breast cancer for subgroups of women defined by reproductive history and other factors; and (5) to determine whether the prescription of OCs is influenced by cancer risk factors such as family history of breast cancer or occurrence of benign breast disease. Since the study data are population -based, age-specific cancer incidence curves will be computed for groups of women defined by OC use and other factors to examine the possibility of age-specific promotional or retarding effects of OC use, or other possible etiologic mechanisms. Finally, estimates of cumulative cancer incidence will be computed to evaluate the overall public health importance of apparently causal relationships.